BEIRUT - Syrian troops attacked rebel positions outside Khan al-Assal on Wednesday as they seek to recapture the northern town at the centre of rival chemical weapons accusations, a watchdog said.

Fierce fighting erupted on the outskirts of the town, which the rebels seized on Monday of last week inflicting heavy losses on the army, the Syrian Observatory for Human Rights said.

The army lost 150 soldiers over two days, 50 of them summarily executed after their capture in an act condemned by the mainstream opposition leadership.

The government has been keen to recapture the town, the last to fall out of its control in the western half of Aleppo province.

The town was the scene of what both the government and the opposition say was a chemical weapons attack that killed 30 people on March 19.

The Syrian government says the rebels carried it out, and its ally Moscow says it has concrete proof.

The opposition says President Bashar al-Assad's regime was behind it and Washington has said it has seen no firm evidence of rebel responsibility.

The Damascus government has pressed for Khan al-Assal to be the primary focus of a planned UN inquiry into a total of 13 allegations of chemical weapons use during the 28-month conflict.

The rebels' capture of the town last week came as two UN envoys visited Damascus to thrash out the ground rules for UN weapons inspectors. They came away with an agreement but UN chief Ban Ki-moon has said he is still reviewing the details.